ON CLAY 215 
as scattered individuals here and there, and bring with them new 
conditions and animal forms. 
a) The subterranean-ground stratum.—In addition to Pyramidula 
mentioned above, other snails appear, especially in the more moist spots 
on the bank. These are Zonitoides, Polygyra monodon (Fig. 169), 
and P. thyroides (Fig. 170). Centipedes (Geophilus) and millipedes 
(Polydesmidae) become more numerous, while the spiders (Pardosa 
lapidicina) (Fig. 168), the tiger-beetle larvae, and other soil-inhabiting 
forms decrease. 
b) Field stratum.—The field stratum of the shrub stage does not 
differ strikingly from the preceding, as it consists mainly of plants of 
the earlier stage scattered among the shrubs. 
c) Shrub stratum.—Here we have the characteristic inhabitants of 
shrubs. On the young aspens and willow are the larvae of the viceroy 
butterfly (163). The common gall on the willow is the pine-cone gall, 
caused by Cecidomyiidae (137). Beneath the leaves of the cone we have 
found long slender eggs of some orthopterous insect (probably Xiphidium 
ensiferum) (40, p. 428). We have no record of the nests of birds, but 
many of the forest margin birds nest here (see pp. 274-75 and Table 
ASI 527 7) 
4. YOUNG FOREST STAGE 
(Fig. 171) 
Shrubs and seedlings of trees become more and more numerous. 
The sweet clover and most of the animals associated with it disappear. 
Young trees, such as oak, hickory, hop, hornbeam, etc., grow and usually 
give rise to a sapling forest. 
a) Subterranean-ground stratum.—This stratum has all the characters 
of the more dense and mesophytic forest ground stratum and largely be- 
cause of the springy.character of the bluff which supplies much moisture. 
The woodchuck (Marmota monax) (142) sometimes digs in these banks. 
In the open places in which small areas of soil are covered with only a 
few leaves we find the larvae of the green forest tiger-beetle (Czcindela 
sexguttata) (55, 151) which lays eggs in shaded places (Figs. 172, 173). 
Under the leaves the snails, which were recorded in the younger stages, 
and sowbugs are present. We find snails and slugs (Polygyra profunda 
[Fig. 220, p. 237] and albolabris [Fig. 240, p. 243], Philomycus caro- 
linensis [Fig. 231, p. 241]), which are commonly abundant in dense 
woods. - The Myriopoda are also more numerous and belong to different 
species. Fontaria corrugate (Fig. 218, p. 237), which has the margins 
